conducting polymers with micro or nanometer structure, 2008, p.307

In 1992, she became a professor and leaded a group to study conducting polymers of polyaniline with regard to the mechanism of proton doping, electrical, opticand magnetic properties and

Conducting Polymers with Micro or Nanometer Structure

Conducting Polymers with Micro or

Nanometer Structure

With 106 figures

Prof Meixiang Wan

Institute of Chemistry, Chinese Academy

of Sciences Beijing, P R China, 100080

E-mail: wanmx@iccas.ac.cn

_

ISBN 978-7-302-17476-9 Tsinghua University Press, Beijing

ISBN 978-3-540-69322-2 Springer Berlin Heidelberg New York

e ISBN 978-3-540-69323-9 Springer Berlin Heidelberg New York

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Pennsylvania, USA, who hadpassed away in 2007.

Author visited Professor MacDiarmid at his office, University of Pennsylvania, USA in 2004

About Author

Meixiang Wan was born in Jiangxi Province, China, in 1940 and graduated fromDepartment of Physics at University of Science and Technology of China in 1965.She joined the Institute of Chemistry, Chinese Academy of Sciences in 1972 inthe Laboratory of Organic Solid State established by Prof Renyuan Qian, anacademy member of Chinese Academy of Sciences, to study electrical properties

of organic solid states including organic photo-conductor, conductor and conductingpolymers In 1985, as a post-doctor she was fortunately recommended byProfessor Qian to further pursure advanced studies on conducting polymers(e g polyacetylene and polyaniline) under Prof Alan MacDiarmid', who wasawarded the Nobel Prize for Chemistry in 2000, at the University of Pennsylvania,Philadelphia, USA

Since returning to China in 1988, she has studied conducting polymers inJapan, France and the United States for a short time (3 - 6 months) as a visitingprofessor and often attends a variety of international conferences in the world In

1992, she became a professor and leaded a group to study conducting polymers

of polyaniline with regard to the mechanism of proton doping, electrical, opticand magnetic properties and related mechanism as well as application ofelectro-magnetic functionalized materials such as the microwave absorbingmaterials In addition, she studied the origin of intrinsic ferromagnetic properties

of organic ferromagnets In 1988, she discovered that conductive nanotubes ofpolyaniline could be synthesized by in-situ doping polymerization in the presence

of B-naphthlene surfonaic acid as a dopant, without using any membrane as ahard-template This novel method is referred to as the template-free method due

to the absence of a membrane as a template Since discovery of the new method,her research has focused on nanostructures of conducting polymers, especiallysynthesized by a template-free method So far, more than 200 papers have beenpublished inAdvanced Materials, Chemical Materials, Micromolecules, Langmuir,

and some of them have been cited for more than 2000 times Moreover, eight bookschapters written in Chinese have been published, including"Conducting Polymer

and Nanotechnology edited by H S Nalwa, America Scientific Publisher in 2004.

In addition, ten Chinese patents were granted and several prizes, such as the Prize

of the National Natural Sciences of China (second degree, 1988), the Prize ofAdvanced Technology of the Chinese Academy of Sciences (second degree,1989), the Prize of Natural Sciences of the Chineses Academy of Sciences (firstdegree, 1995), Outstanding Younger Scientists of Chinese Academy of Sciences(1996) and Excellent Doctoral Teachers of the Chinese Academy of Sciences(2005) were awarded

A traditional idea is that organic polymer is regarded as an excellent insulatorbecause of its saturated macromolecule However, a breakthrough of organicpolymer imitating a metal was coming-out in the 1960s-1970s It implied electrons

in polymers need to be free to move and not bound to the atoms The breakthroughwas realized by awarders of Nobel Chemistry Prizes in 2000, who were Alan

J Heeger at the University of California at Santa Barbara, USA, Alan

G MacDiarmid at the University of Pennsylvania, Philadelphia, USA, and HidekiShirakawa at the University of Tsukuba, Japan In 1977, actually, they accidentallydiscovered that room-temperature conductivity of conjugated polyacetylenedoped with iodine was as high as 103S/cm, which was enhanced by 1010 timescompared with original insulating polyacetylene The change of the electricalproperties from insulator to conductor was subsequently ascribed to "doping", butcompletely differentfrom the doping concept as applied in inorganic semiconductors.The unexpected discovery not only shattered a traditional idea that organicpolymers are insulators, but also established a new filed of conducting polymers

or "synthetic metals"

Since discovery of the first conducting polymer (i.e polyacetylene), conductingpolymers have been received considerable attention because of their uniqueproperties such as highly-conjugated chain structure, covering whole insulator-semiconductor-metal region of electrical properties, a reversible doping/de-dopingprocess, an unusual conducting mechanism and the control of physical properties

by the doping/de-doping process The unique properties not only lead to promisingapplications in technology, but also hold an important position in materialsciences Up to date, the potential applications of conducting polymers includeelectronic devices (e.g Schottky rectifier, field-effect transistor, light emitting diodeand solar cell), electromagnetic interference shielding and microwave absorbingmaterials, rechargeable batteries and supercapacitors, electrochromic devices,sensors (e.g gas, chemical and biochemical sensors) and artificial muscles As aresult, research on conducting polymers has spread rapidly from the United States

conducting and transport mechanisms, processability, structure-property relationshipand related mechanisms as well as applications have been achieved After 23 years,conducting polymers, awarded the 2000 Nobel Prize in Chemistry, have affirmedcontributions of the above-mentioned three scientists for the discovery anddevelopment of conductive polymers, and also for further promoting thedevelopment of conducting polymers.

Proferssor Renyuan Qian as an academy member of the Chinese Academy ofSciences, for the first time, established a laboratory of entilted organic solid state

at the Institue of Chemistry, Chinese Academy of Sciences in the early 1980s Theresearch has covered synthetic method, structureal characterization, and the optical,electrical and magnetic properties and related mechanisms of organic solid statesphotoconductors, conductors, superconductors and ferromagnets as well asconducting polymers

I was fortunate to enter the laboratory recommended by Professor Qian, to studyelectrical properties of organic photoconductors, conductors and conductingpolymers In 1985, I was again recommended by Professor Qian to pursureadvanced studing on conducting polymers under Professor MacDiarmid as apost-doctor In USA, I studied photo-electro-chemistry of polyaniline, which wasdiscovered by Professor MacDiarmid in 1985 for the first time Compared withother conducting polymers, polyaniline is advantageous of simple and low costsynthesis, high conductivity and stability, special proton doping mechanism andcontrolling physical properties by both oxidation and protonation state, resulting

in a special position in the field of conducting polymers These novel physicalproperties and promissing potential applicationsintechnology therefore promissed

me to study continuously polyaniline when I came back from USA to China

in 1988

Since discovery of carbon nanotube in 1991, nanoscience and nanotechnologyhave become some of the fastest growing and most dynamic areas of research inthezo" centrury Scientifically, "nano" is a scale unite that means 1 nanomerter,one billionth of a meter (10-9m) Generally speaking, therefore, the nanomaterialsare defined structural features in the range of 1- 100 nm Based on the definition,

it is understood that nanotechnology deals with atomic and molecular scalefunctional structures With nano-scaled features but large surface area, nanomaterialsoffer unique and entirely different properties compared with their bulk materials.Thereby, the unique properties of nanomaterials result in nanomateials andnanotechnology rapidly spreading to academic institutes and industries aroundthe world

In the 1990s, I accidentally found that nanotubes of polyaniline could be

prepared by a conventional in-situ doping polymerization in the presence of

nanphthalene surfonic acid as the dopant without using any membrane as thetemplate The created method was latterly called as template-free method because

of omitting membrane as a template Especially, further studies demonstrated that

composed of dopnat, dopant/monomer salt or supermolecules even monomer itselfare served as the soft-templates in the formation of the template-free synthesizednanostructures of conducting polymers Compared with the template-synthesismethod, which was commonly used, the efficient and controlled approach toprepare conducting polymer nanostructures is simple and inexpensive because ofthe lack of template and the post-treatment of template removal However, manyquestions dealing with this method were completely un-understood at that time.For instance, how about the universality of the method to nanostructures ofconducting polymers? What is formation mechanism of the self-assemblednanostructures by the method? How about controllability of the morphology andsize for the template-free synthesized nanostructures? Do the electrical properties

of the template-free synthesized nanostructures differ from the bulk materials? Is

it possible to fabricate multi-functionalized nanostructures of conducting polymersbased on template-free method? Can we identify applications for the template-freesynthesized nanostructures? All above-mentioned issues promised me tosystematically and significantly study nanostructures of conducting polymers by atemplate- free method

In fact, the significant progress on conducting polymer nanostructures by themethod has been achieved In 2006, Tsingua University Press in Beijing andSpringer-Verlag GmbH in Berlin invited me to write a book about conductingpolymers and related nanostructures Although a lot of good books and excellentreviews on conducting polymers and corresponding nanostructures have beenwidely published in the world, I was eager to share my knowledge and experience

on studying conducting polymers and their nanostructures with other scientists,teachers and students who are interested in conducting polymers I therefore waspleased to accept the invitation to write up this book

The book consists of five chapters The first chapter briefly introduces basicknowledge of conducting polymers, such as doping item, conducting mechanism,structural characteristics and physical properties of conducting polymers Thesecond chapter further considers structural characteristic, doping mechanism,processability and structure-property relationship of conducting polymers usingpolyaniline as an example The third chapter mainly reviews physical propertiesand corresponding potential application of conducting polymers in technology.The fourth chapter summarizes progress and developing directions in conductingpolymer nanostructures, dealing with synthesis method, unique properties andfabricating technology of nano-arrays, patents, and potential application intechnology The fifth chapter mainly reviews results on template-free synthesizedconducting polymer micro/nanostructures focusingon the universality,controllabilityand formation mechanism of the method, multifunctional nanostructure based ontemplate-free method associated with other approaches, electrical and transportproperties of the self-assembled nanostructures, especially electrical and transportproperties of a single nano-tube or hollow sphere, as measured by a four-probe

guided by reversible wettability I hope this book is able to provide some basicand essential reference information for those studying conducting polymers andtheir nanostructures.

I am very grateful to Professor Alan G MacDiarmid and Renyuan Qian forbringing me to enter the field of conducting polymers I am benefited lifelong fortheir keep improving and conscientiously in sciences Although both ProfessorAlan G MacDiarmid and Renyuan Qian as my kindness teachers have passedaway, their early influence and mentoring are deeply appreciated I sincerelythank all my coworkers and students for their excellent contributions to this book

I especially express my sincere gratitude to my father and mother for their rearkindness, and to my husband, son, and daughter for their love as well as torelatives and friends for their help and friendship

Chapter 1 Introduction of Conducting Polymers 1

1.2 Structural Characteristics and Doping Concept 41.3 Charge Carriers and Conducting Mechanism 7References 13Chapter 2 Polyaniline as A Promising Conducting Polymer 16

2.3.2 Electrical and Charge Transport Properties 27

3.2 EMI Shielding and Microwave Absorbing Materials 55

3.2.2 Microwave Absorption Materials (Stealth Materials) 583.3 Rechargeable Batteries and Supercapacitors 61

3.5.2 Conducting Polymer-Based Artificial Muscles 72

from ID-Nanofibers 2215.3 Mono-Dispersed and Oriented Micro/Nanostructures 2295.3.1 Template-Free Method Combined with Al203Template

5.3.2 Template-Free Method Associated with A Deposition

to Mono-Dispersed and Oriented Microspheres 2315.4 Electrical and Transport Properties of Conducting Polymer

5.4.2 Temperature Dependence of Conductivity 2395.4.3 Electrical Properties ofA Single Micro/Nanostructure 242

5.5.3 Water-Assisted Fabrication of PANI-DBSA Honeycomb

5.5.4 Reversed Micro-Emulsion Polymerization 2525.6 Potential Applications of Conducting Polymer with

5.6.3 Conducting Polymer Nanostructure-Based Sensors

Appendix Term Definitions 278

According to electrical properties, materials can be divided into four-types: insulator, semiconductor, conductor and superconductor In general, a material with a conductivity less than 107S/cm is regarded as an insulator A material with conductivity larger than 103S/cm is called as a metal whereas the conductivity of

a semiconductor is in a range of 104 10 S/cm depending upon doping degree Organic polymers usually are described by V (sigma) bonds and S bonds TheV

bonds are fixed and immobile due to forming the covalent bonds between the carbon atoms On the other hand, the S-electrons in a conjugated polymers are relatively localised, unlike the Velectrons Plastics are typical organic polymers with saturated macromolecules and are generally used as excellent electrical insulators Since discovery of conductive polyacelene (PA) doped with iodine [1],

a new field of conducting polymers, which is also called as “synthetic metals”, has been established and earned the Nobel Prize in Chemistry in 2000 [2] Nowadays, conducting polymers as functionalized materials hold a special and an important position in the field of material sciences In this Chapter, discovery, doping concept, structural characteristics, charge transport and conducting mechanism for the conducting polymers will be brief discussed

1.1 Discovery of Conducting Polymers

In the 1960s—1970s, a breakthrough, polymer becoming electrically conductive, was coming-out The breakthrough implied that a polymer has to imitate a metal, which means that electrons in polymers need to be free to move and not bound to the atoms In principle, an oxidation or reduction process is often accommpanied with adding or withdrawing of electrons, suggesting an electron can be removed from a material through oxidation or introduced into a material through reduction Above idea implies that a polymer might be electrically conductive by withdrawing electron through oxidation (i.e a “hole”) or by adding electron through reduction, which process was latterly described by an item of “doping” The breakthrough was realized by three awarders of Chemistry Nobel Prize in 2000, who were Alan J Heeger at the University of California at Santa Barbara, USA, Alan G MacDiarmid at the University of Pennsylvania, Philadelphia, USA, and Hideki Shirakawa at the University of Tsukuba, Japan [2] In 1977, they accidentally discovered that insulating S-conjugated PA could become conductor with a

conductivity of 103 S/cm by iodine doping [1] The unexpected discovery not only broken a traditional concept, which organic polymers were only regarded as the insulators, but also establishing a new filed of conducting polymers, which also called as “Synthetic Metals” According to a report of the Royal Swedish Academy of Sciences in 2000 [2], there was an interesting story about discovery

of the conducting polymers Since accidental discovery in science often happens, author would like to briefly introduce the story to share with readers Based on above idea of polymer imitating a metal, scientists thought that PA could be regarded as an excellent candidate of polymers to be imitating a metal, because it has alternating double and single bonds, as called conjugated double bonds From Fig 1.1, one can see, PA is a flat molecule with an angle of 120ebetween

the bonds and hence exists in two different forms, the isomers cis-polyacetylene and trans-polyacetylene [2]

Figure 1.1 Molecular structure of polyacetylene [1, 2]

Thereby, synthesis of PA received great of attention at that time At the beginning

of the 1970s, Hedeki Shirakawa at Tokyo Institute of Technology, Japan, was studying the polymerization of acetylene into plastics by using catalyst created

by Ziegler-Natta, who was awarded the 1963 Nobel Prize of Chemistry for a technique of polymerizing ethylene or propylene into plastics Usually, only the form of black powder could be synthesized by using the conventional polymerization method A visiting scientist in Shirakawa’s group tried to synthesize PA in the usual way However, a beautifully lustrous silver colored film, rather than the black powder synthesized by the conventional method, was obtained The unexpected results promissed Shirakawa to check the polymerization conditions again and again, and Shirakawa finally found that the catalyst concentration used was enhanced by 103 times! Shirakawa was stimulated by the accidental discovery and further found the molecular structure of the resulting PA was affected by

reaction temerature, for instance, the silvery film was trans-polyacetylene whereas copper-colored film was almost pure cis-polyacetylene

In another part of the world, chemister Alan G MacDiarmid and physicist Alan

J Heeger at University of Pennsylvania, Philadephia, USA were studing the first metal-like inorganic polymer sulphur nitride ((SN)x), which is the first example

of a covalent polymer without metal atoms [3] In 1975, Prof MacDiarmid visited Tokyo Institute of Technology and gave a talk on (SN)x After his lecture, MacDiarmid met Shirakawa at a coffee break and showed a sample of the golden (SN)x to Shirakawa Consequently, Shirakawa also showed MacDiarmid a sample

of the silvery (CH)x The beautiful silvery film caught the eyes of MacDiarmid and he immediately invited Shirakawa to the University of Pennsylvania in Philadelphia to further study PA Since MacDiarmid and Heeger had found previously that the conductivity of (SN)x could be increased by 10 times after adding bromine to the golden (SN)x material, which is called as “doping” item in inorganic semiconductor Therefore, they decided to add some bromine to the silvery (CH)x films to see what was happen Miracle took place on November 23, 1976! At that day, Shirakawa worked with Dr C.K Chiang, a postdoctoral fellow under Professor Heeger, for measuring the electrical conductivity of PA by a four-probe method Surprise to them, the conductivity of PA was ten million times higher than before adding bromine This day was marked as the first time observed the “doping” effect in conducting polymers In the summer of 1977, Heeger, MacDiarmid, and Shirakawa co-published their discovery in the article entitled “Synthesis of electrically conducting organic polymers: Halogen derivatives of polyacetylene (CH)n ” in The Journal of Chemical Society, Chemical

Communications [1]

After discovery of the conductive PA, fundamental researches dealing with synthesis of new materials, structural characterization, solubility and processability, structure-properties relationship and conducting mechanism of conducting polymers as well as their applications in technology have been widely studied and significant progress have been achieved After 23 years, The Royal Swedish Academy of Sciences has decided to award the Nobel Prize in Chemistry for

2000 jointly to Alan J Heeger at University of California at Santa Barbara, USA, Alan G MacDiarmid at University of Pennsylvania, Philadelphia, USA, and Hideki

Shirakawa at University of Tsukuba, Japan “for the discovery and development

of conductive polymers” [2] Photograph of the three scientists are shown as Fig 1.2 Nowadays, the field of conducting polymers had been well established

Figure 1.2 Photograph of three awardees of the Nobel Chemistry Prize in 2000

Alan G MacDiarmid (left) Prof at the Univ of Pennsylvania, USA Hideki Shirakawa (middle)  Prof Emeritus, Univ of Tsukuba, Japan Alan J Heeger (right) Prof at the Univ of California at Santa Barbara, USA

and conducting polymers as functional materials hold an important position in the field of material sciences Up to date, a large number of articles, reviews and books dealing with conducting polymers has been published Among these books,

“Handbook of Conducting Polymers” (Ed T A Skotheim), Marcel Dekker, New York, 1986 and which re-published in 1998 [4] is a good and basically reference book for scientists and students studying conducting polymers In this Chapter, therefore, only basic knowledge and concepts, such as doping, characteristic of molecular structure, conducting mechanism and electrical and transport properties

of conducting polymers, are briefly discussed

1.2 Structural Characteristics and Doping Concept

Since discovery of conductive PA by iodine doping [1], other S-conjugated polymers, such as polypyrrole (PPy), polyaniline (PANI), polythiophenes (PTH),

poly(p-phenylene)(PPP), poly(p-phenylenevinylene)(PPV), and poly(2,5-thienyl-

enevinylene)(PTV) have been reported as conducting polymers [5], which molecular structure is shown in Fig 1.3 Usually the ground states of conjugated

polymers are divided into degenerate and non-degenerate The prototype of degenerate polymers is trans-polyacetylene, which has alternating CüC and C=C bonds as shown in Fig 1.1 The total energy curve of trans-polyacetylene

has two equal minima, where the alternating CüC and C=C bonds are reversed [1]

On the other hand, a non-degenerate polymer has no two identical structures in the ground state Most conjugated polymers, such as PPy and PANI belong to non-degenerate The band gaps of conjugated polymers are estimated to be typically in the range between 1 and 3 eV from their electronic absorption spectra [4] These observations are consistent with their insulator or semiconductor electrical properties [6] From molecular structure as shown in Fig 1.3, moreover, the polymer backbone in conducting polymers consists of S-conjugated chain, where are the S-electrons of the carbon atoms and the overlap of their wave function The wave overlap is called conjugation, because it leads to a sequence

of alternating double and single bonds, resulting in unpaired electrons delocalized along the polymeric chain [4]

As above-mentioned, PA is the simplest model system for conjugated polymers and is also the first sample for a polymer being conducting polymers [2], indicating S-conjugated polymer chain is a basic requirement for a polymer becoming conducting polymer The delocalization of S-bonded electrons over the polymeric backbone, co-existing with unusual low ionization potentials, and high electron affinities lead to special electrical properties of conducting polymers [7] On the other hand, S-conjugated chain of conducting polymers leads to insoluble and poor mechanical properties of conducting polymers, limiting their application in technology Thereby continue effort to improve solubility and to enhance mechanic strength of conducting polymers is needed

Figure 1.3 Molecular structure of typical conducting polymers

(a) trans-polyacetylene; (b) polythiophenes; (c) poly(p-phenylene); (d) polypyrrole;

(e) poly ( p-phenylenevinylene); (f) poly(2, 5-thienylenevinylene) [5]

As above described, the transition ofS-conjugated polymer from insulator to metal is carried out by a “doping” process However, the “doping” item used in conducting polymers differs significantly from traditional inorganic semicon- ductor [5] Differences in “doping” item between inorganic semiconductors and conducting polymers are shown as follows:

(1) Intrinsic of doping item in conducting polymers is an oxidation ( p-type doping) or reduction (n-type doping) process, rather than atom replacement in inorganic semiconductors Using PA as a sample, for instance, the reaction of p- and n-doping is written as:

Oxidation with halogen ( p-doping): [CH]n 3 / 2Ix 2 [CH]n x xI3

Reduction with alkali metal (n-doping): [CH] n xNa [CH]x n xNa

(2) p-doping (withdrawing electron from polymeric chain) or n-doping (additing

electron into polymeric chain) in conducting polymers can be acquired and consequently accompanied with incoppration of couterion, such as cation for

p-doping or anion for n-doping, into polymer chain to satisfy electrical nature In

the case of oxidation, taking PA as a sample again, the iodine molecule attracts an electron from the PA chain and becomes I3 The PA molecule, now positively charged, is termed a radical cation [1] Based on above description, therefore, conducting polymers not only consist of S-conjugated chain, but also containing counter-ions caused by doping This differs from conventional inorganic semiconductors, where the counterions are absent The special chain structure of

conducting polymers results in their electrical properties being affected by both structure of polymeric chain (i.e S-conjugated length) and dopant nature Doping process can be completed through chemical or electrochemical method [4] Except for chemical or electrochemical doping, other doping methods, such as “photo- doipng” and “charge-injection doping”, are also possible [8] For instance solar cells is based on “photo-doping” whereas light emitting diodes (LEDs) results from “charge-injection doping”, respectively, that are further discussed in Chapter 3 Besides, “proton doping” discovered in PANI is an unusual and efficient doping method in conducting polymers [9] The proton doping does not involve a change in the number of electrons associated with the polymer chain [10] that is different from redox doping (e.g oxidation or reduction doping) where the partial addition ( reduction ) or removal (oxidation ) of electrons to or from the Ssystem of the polymer backbone took place [4,11]

(3) The insulating S-conjugated polymers can be converted to conducting polymers by a chemical or electrochemical doping and which can be consequently recombacked to insulate state by de-doping This suggests that not only de-doping can take place in conducting polymers, but also reversible doping/de-doping process, which is different from inorganic semiconductor where de-doping can’t take place [5] As a result, conductivity of the conducting polymers at room temperature covers whole insulator-semiconductor-metal region by changing doping degree as shown in Fig 1.4 On the contrary, those processes are impossible

to take place in inorganic semiconductors!

Figure 1.4 Conductivity of conducting polymers can cover whole insulator-

semiconductor-metal region by changing doping degree [5]

(4) The doping degree in inorganic semiconductor is very low (~ tenth of thousand) whereas doping degree in conducting polymers can be achieved as high as 50% [5] So electron density in a conducting polymer is higher than that

of inorganic semiconductor; however, the mobility of charge carriers is lower than that of inorganic semiconductor due to defects or poor crystalline

(5) Conducting polymers mostly composed of C, H, O and N elements and their chain structure can be modified by adding substituted groups along the chain or as the side chains that result in conducting polymers reserving light-weight and flexibility of conventional polymers Based on above descriptions, conducting polymers are intrinsic rather than conducting plasters prepared by a

physical mixture of insulating polymers with conducting fillers (e.g carbon or meter) [11] The differences of the conducting plastics from conducting polymers also exhibit as follows: one is the conductivity of conducting plasters increases suddenly at a percolation threshold, at which the conductive phase dispersed in the non-conductive matrix becomes continuous, while conductivity of the conducting polymers increases with increase of the doping degree Another is the conductivity of the conducting plastics is lower than that the doped conducting polymers, for instance, their conductivity of the conducting plastics above percolation threshold is only 0.1 0.5 S/cm at 10 wt%  40 wt% fractions of the conductive filler In addition, the position of the percolation threshold is affected

by particle size and shape of filler [12]

1.3 Charge Carriers and Conducting Mechanism

As is well known, conductivity ( ),V as measured by a four-probe method, is an important property for evaluation of conducting polymers Usually V is expressed

as neP, where e is charge of electron, n and Pare density and mobility of charge carriers, respectively The doping concept in the conducting polymers completely differs from inorganic semiconductors, as above-described, leading to a significant difference in electrical properties between conducting polymers and inorganic semiconductors, which are summarized as follows:

(1) Inorganic semiconductor process few charge carriers, but these carries have high mobility due to the high crystalline degree and purity presented by these materials On the contrary, conducting polymers have a high number of charge carriers due to a large doping degree (>50%), but a low mobility attributed to structural defects

(2) A free-electron is regarded as a charge carrier in a metal; and temperature dependence of the conductivity for a metal increases with decreasing temperature

On the other hand, electron or hole is assigned as a charge carrier in an inorganic semiconductor and the electrical properties of semiconductors are generally

dominated by minion charge carrier (electron or hole) produced by n- or p-type

doping Charge transport in a semiconductor is described by a band model, which the electrical properties are dominated by the width of the energy gap, which is defined as a difference in energy between the valence band and conducting band,

as presented by E The charge transport in semiconductor can be therefore g.expressed by following equation,

temperature, respectively For a conducting polymer, solitons [13], polarons [14]

and bipolarons [14b, 15] are proposed to interpret enhancement of conductivity

of S-conjugated polymers from insulator to metal regime via a doping process Usually, soliton is served as the charge carrier for a degenerated conducting polymer (e.g PA) whereas polaron or bipolaron is used as charge carrier in a non-degenerated conducting polymer (e.g PPy and PANI) [4] The model assumed that soliton can move along the PA backbone carrying charge but no spin (spinless), and if an electron is added to the action or taken away from the anion, a neutral radical soliton is again established [16] In a mechanism involving solitons, electron conduction involves only fully occupied bands in the ground state and leads to formation of a half-occupied electronic level (one electron) within the gap Theoretical models also demonstrate that two radical ions (polarons) react exothermically to produce a dication or dianion (bipolaron) The polaron is thermodynamically more stable than two polarons due to electronic repulsion exhibited by two charges confined in the same site and cause strong lattice distortions Meanwhile, polaron is spin whereas bipolaron is spinless As a result, polaron and bipolaron can be distinguished by means of electron spin response (ESR) Schematic positive polaron and bipolaron as two positive polarons in PTH are as shown in Fig 1.5 The chemical term, charge and spin for soliton, polaron and bipolaron are also given in Table 1.1

Thus charge carrier (i.e soliton, polaron and bipolaron) in conducting polymers

is different from either free-electron in a metal or electron/hole in an inorganic semiconductor It should point out that the item of soliton, polaron and bipolaron

is only used to interpret the electronic motion along the segment of polymeric chain [4] As above-mentioned, the polymeric chain of the doped conducting polymers composes of S-conjugated length and counter-ions, depending upon

Figure 1.5 Schematic structure of (a) a positive polaron, (b) a positive bipolaron,

and (c) two positive bipolarons in polythiophenes [16]

Table 1.1 Chemical term, charge and spin of soliton, polaron and bipolaron in

conducting polymers

Carrier nature Chemical term Charge Spin

Neutral soliton Neutral radical 0 1/2

Positive polaron (hole polaron) Radical cation +e 1/2

Negative polaron (electron polaron) Radical anion  e 1/2 Positive bipolaron Dication +2e 0

doping fashion (e.g cation for n-doping whereas anion for p-doping) Obviously,

conductivity of the conducting polymers is also affected by parameters as follows: (1) Chain structure includes S-conjugated structure and length, crystalline and substituted grounds and bounded fashion to the polymeric chain Regarding polymeric chain structure, for instance, the maximum value of the conductivity

in iodine-doped PA was on the order of 103 S/cm [1, 17] On the other hand, the maximum conductivity for the doped PPy [4] and PTH [4, 18] were below 200 S/cm RegardingS-conjugated length, it is found that high number of conjugated length for a high electrically conductive polymer is unnecessary, because the conductivity

of oligomers is comparable with its long-conjugated polymers as shown in Chapter 3 Regarding crystalline, in general, the electrical conductivity at room temperature is proportional to the crystalline degree because of closer intermolecular distance in crystalline phase [19] Therefore, the conducting polymers with a branched chain have a low conductivity at room temperature is expected due to less crystalline

(2) Dopant structure and doping degree are keys to realize an insulating S-conjugated polymer to become a conducting polymer Molecular structure of dopants not only affects electrical properties, but also solubility in organic solvent or water For example camphorsulfonic acid (CSA) doped PANI not only

has high conductivity (200 S/cm), but also can soluble in m-cresol [20], which will

be discussed in Chapter 3 in detail As shown in Chapter 5, moreover, morphology and diameter of conducting polymer nanostructures prepared by either hard- and soft-template method are strongly affected by dopant nature and dopant degree Regarding doping degree, in general, room-temperature conductivity of the conducting polymers, as measured by four-probe method, is a function of the doping degree, showing the conductivity increases with increase of the doping degree undergoing from insulator to metal through a semiconductor

(3) Polymerization conditions including concentration of monomer, dopant and oxidant, the molar ratio of dopant and oxidant to monomer and polymerization temperature and time are other important parameters affect the conductivity, because these are contributed to chain conformation, morphology and crystalline of

the final product In Chapter 5, the influences of these parameters on morphology, diameter and electrical properties are discussed by using sufficient samples Above-mentioned parameters should keep in mind as one studies conducting polymers even though their nanostructures!

In principle, temperature dependence of the conductivity, as measured by four-probe method, can be used to describe characteristic of charge transport for

a material Temperature dependence of conductivity can be expressed by a logarithmic derivative, D 'lnV/ ln ' T Metal has a positive temperature coefficient of D and a finite dc conductivity as T o is observed On the contrary, 0

D for insulator or semiconductor is a negative coefficient The symbol of Dis therefore can be used to distinguish between metal and semiconductor or insulator Metallic-like conductivity of conducting polymers at room temperature (V~102 103S/cm) has been observed [21] Moreover, the metallic properties of the doped conducting polymers (e g PA) have been revealed by their optical properties [22], thermo-electrical power [23] and magnetic susceptibility [24] Similarly, heavily doped PTH also shows metallic properties, such as Pauli spin susceptibility [25] and a linear temperature dependence of the thermoelectric power have been observed [26] Based on the one-electron band theory, Furukawa [16] suggested that the interaction between polarons in the polaron lattice leads to the formation of a half-filled band responsible for the metallic properties, because the electronic wave function of each polaron in the polaron lattice is overlapped, indicating the electronic states are not localized However, the metallic temperature dependence of the conductivity is not observed instead

of a thermally activated conduction characteristic of a semiconductor, in other word, a negative D coefficient was observed Moreover, finite dc conductivity as 0

T o was also not observed [27] This is attributed to the inter-contact resistance

in the inter-febrile, inter-granular or inter-crystallinity regions of conducting polymers Similar problem has been encountered in the measurement of the temperature dependence of conductivity of polycrystalline powder compactions,

as measured by four-probe method Coleman [28] proposed a voltage shorted compaction (VSC) method could effectively short circuit the inter-crystallinity contact resistance, showing true temperature dependence of conductivity VSC method is similar to four-probe method, for instance, four metallic wires with a same distance are used as the probes However, the specimen between two voltage terminals is shorted by a thin layer of silver paste [29] Author proved the validity

of VSC method by comparison of temperature dependence of conductivity of Qn(TCNQ)2 polycrystalline powder measured by VSC method with that of single crystal measured by four-probe method [30] It showed that temperature dependence of conductivity of Qn(TCNQ)2poly-crystalline powder, as measured

by VSC method, was in agreement with the results obtained from its single crystal, suggesting the intrinsic properties of temperature dependence of the tested materials can be qualitatively determined by VSC method due to eliminate the

inter-crystalline contact resistance However, the specific conductivity, as measured

by VSC, is meaningless, because the measured conductivity involves resistance

of the silver paste Besides, author also proposed a simple physical model to interpret why VSC method can qualitatively be used to determine the intrinsic properties of temperature dependence of conductivity of the test materials [31] The model was assumed that the measured resistance is dominated by resistance

of three layers: resistance of the silver paste itself (presented by A layer), resistance

of the particles of conducting polymers or organic poly-crystals immersed in the silver layer (represented by B layer) and the resistance of the tested materials (presented by C layer) Obviously, the inter-particle contact resistance in the layer B is completely shorted by silver paste Moreover, the resistance of the layer B can be considered as the resistance of the test material and silver paste in series The resistance between voltage terminals of VSC device, therefore, consists

of the resistance of A, B and C layers in parallel

According to mathematical analysis of this model, it is found that the layer B

is a technical key for preparing VSC device and its thickness should be thinner as possible in order to have successful VSC measurement [31] Author, for the first time, successfully observed the metallic temperature dependence of conductivity

of highly-doped PA by VSC method [29], i.e., the conductivity increased with decreasing temperature from 300 to 77 K, exhibiting a metallic behavior Moreover, metal-semiconductor transition for PPy [32] and PTH [33] was also observed by VSC method

The substantial progress in developing new conducting polymers and in enhancement of conductivity were obviously for the last thirty years In 1987, for example, Naarmann et al [34] reported that the conductivity of doped PA was as high as of 104 S/cm comparable to those of traditional metals (e.g lead), showing the onset of a new generation of conducting polymers After that, the conductivity

of doped PA continually increased to 105 S/cm reported by Tsukomoto [35] In addition, conductivity of doped PPV on order of 104 S/cm was also reported [36] Moreover, the PF6-doped PPy synthesized by electrochemical polymerization at

a lower temperature showed a high conductivity (~ 103 S/cm) and, for the first time, a positive temperature coefficient of resistivity (TCR) was observed at temperature below 20 K [37] Meanwhile the conductivity of CSA doped PANI was

as high as 300 400 S/cm [20] and a significant positive TCR in the temperature range 160 300 K [38] was also observed Although a great progress has been made in enhancement of room-temperature conductivity of the conducting polymers,

as above-mentioned, it is necessary to reduce the microscopic and macroscopic disorder and thereby bring out the intrinsic metallic features of conducting polymers

Up to date, various models have been proposed to interpret charge transport of conducting polymers For instance a model considered charge transport by inter-chain hopping has been proposed by Matveeva [39] This model suggested

that single chain or intra-molecular transport, across inter-chain transport and inter-particle contact contribute to the overall conductivity of the conducting polymers, as measured by four-probe method It implies that these elements comprise a complicated resistive network, which determines the effective mobility

of the carriers in the conducting polymers This model also indicated that the mobility and the conductivity of conducting polymers are dominated by both microscopic (intra-chain and inter-chain) and macroscopic (inter-particles) level [40] Although room-temperature conductivity of the conducting polymers shows a metal-like behavior, charge transport across inter-chain and inter-particles in conducting polymers, as presented by temperature dependence of conductivity, only exhibits a semiconductor behavior and obeys a Variable Range Hopping (VRH) model proposed by Mott and Davis [41], which is expressed as

0T n exp[ (T T0/ ) n ]

 (1.4) where V0 and T are constants and 0 n 1, 2, and 3 for 3D, 2D and 1D

conduction, respectively The value of “n” is defined as the dimensionality of the

conduction, which can be obtained from the plot of log V against to 1/( 1)

n

TheMott parameter ( )T is directly proportional to the density of state at Fermi level 0

and is inversely proportional to the location length [41] It is found that the

dimensionality (n) is affected by the diameter of nanostructure, for instance,

3D-electronic conduction in PANI or PPy nanotubes with a 400 nm is often observed [42] whereas 1D-conduction is occurring for small diameter tubules owing to the large proportion of the ordered material [43]

Sheng and Klasfter’s fluctuation induced tunneling (FIT) model has been also widely used to interpret temperature dependence of conductivity of conducting polymers (e.g high conducting PA) [44] However, some argues on the FIT model were arisen [45] and modification on the FIT model was also proposed [46] Baughman and Shacklette [47] proposed a random resistor net-work model with

a wide distribution of activation energies to interpret temperature dependence of conductivity for conducting polymers A similar correction between the conjugation length, conductivity, and the exponential temperature dependence of  ( )V T was reported by Roth [48] Epstein et al [49] also proposed a “metallic islands” model, which is a composite model consisting of high conductivity crystalline regions surrounded by insulating amorphous regions, to interpret the transport properties of PANI protonated by common acids (e.g HCl and H2SO4) Besides, some other models, such as random dimmer model with a set of delocalized conducting states [50] and polaron/bipolaron model involving correlated hopping and multi-phonon process [51] have been also proposed for explaining the exponential temperature dependence of conductivity observed in conducting polymers

Based on above-discussions, in summary, conducting polymers not only have semiconductor and metal properties, but also maintain many advantages of

conventional polymers, such as light weight, flexible chain and processability The unique properties resulted from delocalized S-conjugated structure and unusual doping concept and reversible doping/dedoping process, and conducting mechanism completely differs from either inorganic semiconductors or metals Moreover, it should be noted that the conducting polymers are intrinsic and differ from conducting plastics blended by conductive carbon or metallic materials as the fillers.

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As mentioned in Chapter 1, the discovery of the conducting polymers was attributed

to MacDiarmid et al who exposed free standing films of PA to vapors of chlorine, bromine and iodine, resulting in increasing 12 orders of magnitude in conductivity

at room temperature [1] In fact, the first work describing the synthesis of what is now recognized as a conducting polymer was “Aniline black” published in

1862 [2] “Aniline black” was prepared from the anodic oxidation of aniline and accompanied a color change upon switching potential, which latterly was called

as electrochromism [3] However, it was regret that the electrical properties were not measured at that time! In 1985, MacDiarmid, for the first time, found that aniline monomer in an acid aqueous solution (e.g.1.0 mol/L HCl) can be chemically oxidized by ammonium peroxydisulfate (APS) to obtain green powder of PANI with a conductivity of as high as 3 S/cm, as measured by four-probe method, which results were published in 1986 [4].This was the first sample of the conducting polymers doped by proton, which latterly was called “proton doping” Compared with other conducting polymers, PANI is advantageous of easy synthesis, low-cost, structure complex and special proton doping mechanism, as well as physical properties controlled by both oxidation and protonation state These unique properties result in PANI holding an important position in the field of conducting polymers At that time, author was pursure advanced studying on conducting polymers under Prof MacDiarmid at University of Pennsylvania, the United States of America as a post-doctor As a result, author was fascinated by the novel properties and promising application of PANI to further study PANI when came back to China from USA in 1988 The research objects included synthesis method, doping mechanism, solubility, and physical properties of PANI In this chapter, therefore, author would like to further introduce character of molecular structure, synthesis method, doping concept and associated conducting mechanism and processability of conducting polymers by using PANI as a promising sample

2.1 Molecular Structure and Proton Doping

Compared with other conducting polymers, PANI has a complex molecular structure dominated by its oxidation and protonation state MacDiarmid et al [5], for the first time, proposed that the base form of PANI has a general formula [6]

as shown Fig 2.1 They assumed that the base form of PANI consists of alternating reduce and oxidized repeat unit chain and can be divided into three states based on the oxidation state [4, 7] as shown in Fig 2.1 The completely reduced form and oxidized form are assigned as “leucoemeraldine” base form (y LEB) and “pernigraniline” form (1, y PEN), respectively The “half- 0,oxidized” form is called as “emeraldine base” form (y 0.5,EB) The general molecular formula of the base form of PANI proposed by MacDiarmid has been firstly conformed by the analysis of the 13C-NMR spectra [8]

Figure 2.1 (a) Generalized composition of PANI indicating the reduced and oxidized

repeat units; (b) completely reduced polymer; (c) half-oxidized polymer; (d) fully oxidized polymer [6]

As above-mentioned, PANI was the first sample of a doping conjugated polymer to a highly conducting regime by a proton doping [4] Proton doping means that the emeraldine base form (EB, y 0.5) is doped with a protonic acid (e.g 1.0 mol/L HCl) to produce a protonated emeraldine base form with a high conductivity (~ 3 S/cm), which is called as the emeraldine salt form (ES) [4, 7b]

As shown in Fig 2.2, the proton doping dose not involve a change of the number

of electrons associated with the polymer backbone during the proton doping [4, 7b] This significantly differs from redox doping (e.g oxidation or deduction) where involves the partial addition (reduction) or removal (oxidation) of electrons to or from the polymer backbone [9] Thus proton doping is major characteristics of PANI differing from other conducting polymers

Figure 2.2 Scheme of proton doping in PANI [6]

In principle, the imine nitrogen atom on the polymeric chain of PANI can be protonated in whole or in part to obtain the corresponding salts and the protonation degree on the polymeric base, depending on its oxidation state and the pH value

of the aqueous acid [4] MacDiarmid et al [5], for the first time, proposed that proton doping only took place on the imine segment of the emeraldine base form

to form a bipolaron However, this was contradictory with both theoretical calculation and ESR, because theoretical calculation rules out the presence of a bipolaron lattice (spinless) in the emeraldine salt form [10] and a strong ESR signal rather than spinless was observed [11] In order to solve above contradictory, Epstein et al [11, 12] and Wnek [13] suggested that spinless bipolaron can convert to two spinning protons due to instability of bipolaron MacDiarmid et

al [7b] also proposed the polaron is of semiquinone form as shown in Fig 2.2 It means that the complete protonation of the imine nitrogen atom in the emeraldine base by proton doping results in the formation of a delocalized poly-semiquinone radical cation [4, 7b, 9a] Author also studied mechanism of proton doping in PANI

by means of UV-visible and fluorescence spectra as well as in situ UV-visible

and ESR spectra [14] The main results are summarized as follows:

(1) Further proved MacDiarmid suggestion [4] that the protonation process only takes place on the imine segment of the poly-emeraldine chain

(2) Proposed that the protonation process consists of a chemical reaction of proton with nitrogen on the imine segment and a diffusion process of counter-irons from aqueous solution to polymers The first process is fast whereas the later is a slow process

(3) Suggested that the protonation state (i.e the emeraldine salt form, ES) should be considered in molecular structure of a partial protonated state This

implies molecular structure of the protonated PANI should be determined by both oxidation and protonation state This was further conformed by UV-visible spectral

of thin film of PANI with different protonation state (i.e different pH) [15] As shown in Fig 2.3, VU-visible spectra for the emeraldine base form show two peaks at 325 and 630 nm (Fig 2.3) which are assigned as the excitation of the amine and imine segment on the polyemeraldine chain, respectively [10, 16] This is consistent with that the molecular structure of the emeraldine base form

of PANI consists of an amine and imine segment on the polyemeraldine chain [5] For fully protonated emeraldine base form, on the other hand, the peak at 630 nm disappeared, whereas a new peak at 950 nm occurred that is assigned as proton band produced by proton doping [7b, 10 12] In addition, the position of the peak at 325 nm does not change with its protonation state although the intensity

of this peak decreases with its protonation state (Fig 2.3) [15] These indicated that the protonation process only takes place on the imine segment of the emeraldine chain, which was also consistent with Huang’s suggestion [4] That was also conformed by fluorescence excitation and emission spectra of PANI [14] It noted that the intensity of peaks at 630 and 950 nm are strongly affected by the protonation state [15] As shown in Fig 2.4, the intensity of the peak at 630 nm decreases with increase of the protonation state (i.e reduced pH value).On the contrary, the intensity of the peak at 950 nm increases with increase of the protonation state; however, the peak at 630 nm is still observed in

a partially protonated PANI (e.g at pH 2 6),indicating the quinoid segment

on the polyemeraldine chain is present in partially protonated PANI Based on above results, author modified the general molecular formula of PANI by adding

a parameter of the protonation state (x) as shown in Fig 2.5 [15]

Above-described molecular structure and protonated state of APNI are also supported by its FTIR spectra For example, the C=C stretching deformation of quinoid (1570 cm1) and benzenoid rings (1494 cm1), the CüN stretching of

Figure 2.3 Absorption of the emeraldine base form of PANI files with different

protonation state (i.e pH) [15]

Figure 2.4 Intensity of the peak at 630 and 950 nm as a function of the

1141 cm1 related to proton doping PANI are observed [17] Similar to other conducting polymers, the emeraldine salt form of PANI (i.e conducting state) can be converted to its emeraldine base form (i.e insulating state) by a base (e.g NaOH or ammonium), consequently, which can be re-doping by acids to form the emeraldine salt form, showing a reversible protonation/de-protonation process [7b]

Except for proton doping, PANI is also able to be p-doped by charge transfer

with an oxidation agent, achieving conductivity up to about 1.0 S/cm [4, 7b, 18]

In particular, Chen et al [19] reported a novel soluble n-doped PANI in dimethyl

suffixed (DMSO) synthesized by doping the emeraldine base of PANI with a strong reductant (KH or NaH) The room-temperature conductivity, spin density

and magnetic susceptibility of the n-doped PANI were comparable with self-

doping PANI [19] However, stability of the conductivity is poor, for instance, the conductivity decreases rapidly by 6 7 orders of magnitude within 90 s as

exposed to air In addition, they also suggested that the n-doping can be considered

as a direct doping on the full oxidized form of PANI (i.e PEN) by using the

alkali metals K and Na [19] Besides, n-doped and 100% sulfonated PANI,

which is soluble in both organic and aqueous solvent, has been also synthesized

by electrochemical polymerization in an acetoneeitrile-water mixture (4:1) [20]

In particular, PANI can be doped by a photo-induced doping In this method, photo-acid generators, which can release protons under light excitation, are required In other words, the proton released by photo-acid generators under light excitation is served as a dopant Based on above mechanism, Marier et al [21] extensively studied on structure-property relationship of the PANI doped by photo-acid generators Author also obtained a conducting PANI film by using a copolymer of vinylidene chloride and methyl acrylate (VCMAC) as a photo-acid generator [22] Moreover, Lee et al [23] reported a soluble PANI synthesized by

modifying with a photo-labile, acid-labile, and thermo-labile tert-butoxycarbonyl (t-BOC) group The resultant PANI (t-BOC) is highly soluble and thermody-

namically stable in low-boiling solvents (e.g THF, dioxane, and CHCl3) and

suggested the PANI (t-BOC) can be used as conductive matrix polymers for

negative type photo-imaging or printing materials or for novel solution-processed applications in various microelectronic devices, which more discussions are given in Chapter 3

PANI prepared by MacDiarmid method is mostly amorphous [7b], as measured

by X-ray diffraction (XRD) Usually, the XRD of the doped PANI shows a broad peak centered at 2T 18e and 25ethat are attributed to the periodicity perpen- dicular and parallel to the polymer chain, respectively [24] Except for the long- chain PANI, in addition, it also has its oligo-aniline, which electrical properties are as same as its long-chain [25] The oligo-aniline is generally divided into three groups according to their terminal groups, which include amino-capped, phenyl- capped and amino/phenyl-capped [26], as synthesized by a simple oxidative coupling reaction [27]

Like other conducting polymers, PANI is generally synthesized by both chemical and electrochemical methods [9a] But mechanochemical route has been recently reported to prepare conducting PANI These methods to PANI will be briefly discussed below

2.2.1 Chemical Method

In general, the emeraldine salt form of PANI (i.e its conducting state) is prepared

by oxidative polymerization of aniline in a strong acidic medium [4,5, 18a] The scheme of chemical polymerization of aniline can be described as follows:

4 3

NH OH Oxidant

Typical chemical polymerization of PANI is as follows [9a]: APS of 10 mL (e.g of 2.5 mol/L) as an oxidant is added with stirring to 100 mL solution of

a mixture of aniline monomer (e.g 0.55 mol/L) and 1.0 mol/L HCl at room temperature or in an ice/water bath (0 5ć) and the polymerization is continued for pre-desired time The precipitate is then filtered, washed with small amounts

of 0.5 mol/L HCl solution, then with methanol until colorless, and finally diethyl ether until color-less It is finally dried at ca 60 70ć in air for ca 24 h The product so obtained is the emeraldine salt form with conductivity of ~ 10 S/cm, depending preparation conditions The resultant emeraldine salt form can be converted to its emeraldine base form by NH4OH or NH3, which process is called

as de-doping From above scheme of polymerization of aniline, one can see, the reaction system of aniline polymerization is very simple, only including monomer, dopant and oxidant and water Among these reagents, dopant is the most important reagent, because it is mainly attributed to electrical properties of PANI Speaking generally, inorganic acids (e.g HCl, H2SO4, H3PO4 and HF) and organic acids are widely used as dopants for doping PANI Dopant feature includes molecular structure and acidity, strongly affect the properties of the doped PANI [9a] Except for dopant, oxidant is another important reagent for chemical polymerization

of PANI Up to date various oxidants such as APS [4], tetrabutylammonium persulfate (TBAP) [28], hydrogen peroxide (H2O2) [29], benzoyl peroxide [30], ferric chloride (FeCl3) [31] and chloroaurate acid (HAuCl4) [32], have been used

to synthesize PANI However, most previous reported results mainly emphasized the effect of oxidants on the polymerization yield, and APS was regarded as the optimal oxidant for PANI because of its high yield [33] Interestingly, author found that the diameter of PANI nanotubes synthesized by template-free method

is directly related to the redox potential of oxidant [34] that will be discussed in Chapter 5 in more detail In addition, reaction temperature, polymerization time

as well as stirring fashion and speed also affect the structure and physical properties of the final PANI product Reaction temperature is more important because it affects the crystallinity of PANI, showing a low reaction temperature

is favorable for preparing high crystalline PANI [9a] A low reaction temperature can be achieved by freezing the reaction bath (e.g ice/salt bath with T<4ć) or

by addition of inorganic salts (e.g NaCl) in the reaction solution, resulting in the polymerization is proceed in the solid state [35]

2.2.2 Electro-Chemical Method

Electrochemical polymerization is another major tool to prepare conductive PANI

In an electrochemical method, a PANI film or layer is formed on a work electrode surface in an acidic medium (e.g 1.0 mol/L HCl) including high concentrations

of aniline monomer [9a] The electrochemical method has some advantages over

chemical procedures to prepare PANI as follows: ķ Electrochemical method is easy to be controlled by changing current quantity passed through between the work and counter electrodes, applied voltage and polymerization time Moreover, the oxidation and electrically conducting form of PANI is reversibly transformed into the reduced, isolating form by changing its electrochemical potential to negative values On the other hand, the chemical method is difficult to ensure a same micro-environment for preparing PANI, insulting in disorder of the product

ĸ A small amount reagent is required in an electrochemical method On the contrary, a large amount of reactive chemical reagents in the chemical method is required, which could cause environment pollution ĹThe quantum of the product prepared by electrochemical method is limited by size of the work electrode; however, large amount of product is easily synthesized by a chemical method It has been demonstrated that the electrochemical polymerization of aniline is as a bimolecular reaction involving a radical cation intermediate and a two-electron-transfer process for each step of polymerization [36] Based on the assumption that the oxidation of PANI follows Faraday’s law, the electrochemical polymerization of aniline can be monitored by PANI deposition on the work electrode and doping charges [37] In some strong acids such as HC1 or H2SO4,the deposition charge was found to increase proportionally to the second powers

of the cycle number [36] When potentiostatic techniques was used to electro- chemically polymerize aniline, moreover, the initial formation (or nucleation) of PANI on the surface of the electrode is slow, but the initially formed PANI accelerated the polymer growth greatly and the current increased proportionally with the second power, which is called as “self-catalyzing” or “auto-acceleration” process in the electrochemical polymerization of aniline [38] Wei et al [36] employed the anodic peek current instead of charges to monitor the electrochemical polymerization process of aniline in aqueous HCl solution using cyclic potential sweep techniques This method was more sensitive to the presence of irreversible side reaction [39] By using this method, the cyclic voltammograms of aniline polymerization showed three peaks [36], which are associated with three oxidation states suggested by MacDiarmid [6] Wei et al [40] also investigated effect of the additives, such as paminodiphenylamine, benzidine, and phenoxyaniline,

on the rate of electrochemical polymerization of aniline They found all of these additives have lower oxidation potentials than the aniline monomers and at least one satirically accessible aromatic amino group incorporated into polymer chains

as a part of the structural backbones [40]

2.2.3 Mechano-Chemical Route

Since the liquid monomer aniline forms solid salts with doping acids (e.g HCl,

H2SO4 and CSA) through an acid/base reaction, room-temperature solid-state polymerization of aniline is possible using a solid anilinium salt as the precursor

Kaner et al [41] recently describe a solvent-free mechanochemical route to PANI in which the reaction is induced by ball-milling an aniline salt and an oxidant under ambient conditions According to the report of Kaner et al [41], the typical synthesis procedure of mechanochemical route to PANI is shown as: firstly a salt of aniline and CSA was prepared by the reaction between aniline and CSA in water followed by evaporation that was served as the precursor Then a mixture of an anilinium salt (e.g aniline/CSA) and APS as an oxidant were loaded into a stainless steel grinding bowl and which was then sealed and spun at

600 r/min in a planetary micro-mill for 1 h [41] Once the spinning stopped, the product was transferred into a beaker and washed with water for several times A yield of up to 65% based on aniline can be achieved using a 1:1 molar ratio of anilinium salt to oxidant and typical sample of the PANI powder prepared by this method is shown in Fig 2.6 Spectroscopic studies indicate that the resultant PANI is identical to the emeraldine salt form of PANI with a conductivity of

102 S/cm [41] This method opens a sample and solvent-free method to prepare

a large amount of the conducting PANI

Figure 2.6 Powder of PANI prepared by mechanochemical method for 1 h [41]

Like other conducting polymers, PANI has unique physical properties including optical, electrical and magnetic properties and corresponding effects (Schottky and thermo-chromic effect and photo-emission effect) which will be briefly discussed below

2.3.1 Nonlinear Optical (NLO)

Nonlinear optical (NLO) effect is defined that when a light passes through a nonlinear optic-active medium, the frequency, phase, amplitude or transmission

of the coming out of light are changed [42] The NLO phenomena therefore are expected to provide hey functions necessary for the photonic technology of optical switching, frequency modulation, wave-guiding, and eventually practical all-optical computing [43].ҏ Compared with inorganic or organic NLO materials, NLO polymers are of somewhat advantageous as follows: ķ The design, synthesis and fabrication of NLO polymers are more flexible, facile and cost-effective than those of inorganics [44] ĸ The lower dielectric constant of polymers make it easier to design traveling wave electro-optic (EO) modulators due to the close match of velocity between the microwave and optic wave Ĺ Processing polymers

in conjunction with thin films technologies offers the unique opportunity for them to be used in inter-grated optic and EO application As a result, organic and polymeric NLO materials have been received great attention in NLO materials and their applications in technology

Generally speaking, the NLO phenomena can be described at both the molecular and bulk lever The molecular polarization in an intense electric field

is field-dependent and is usually described by equation [45]:

0

P P D E J (2.1) where P is the molecular dipole moment, P0 is the intrinsic dipole moment of

the molecule, and E is the electric field vector The DE and Jcoefficient represents the polarizability, hyperpolarizability and the second hyperpolarizability

of the molecules, respectively The bulk polarization of a material can be described in a similar equation:

(1) (2) 2 (3) 3 0

( )

P E P F EF E F E (2.2)

in which F(1),F(2)andF(3) are assigned as the first-, second- and third-order

susceptibilities and P represents the bulk polarization of the medium P is the 0

permanent polarization of the material Therefore, nonlinear responses in bulk media are usually described by (2)

F and (3)

F , respectively According to above

Eq (2.2), all materials including gases, liquids and solids can show third-order NLO effects, but second-order NLO materials require a noncentrosymmetric alignment

of NLO molecules [45]

Bulk and molecular nonlinear optical properties can be measured by laser optical techniques such as second and third harmonic generation (SHG and THG), electric-induced second harmonic generation (EFISH), and degenerate four-wave mixing (DFWM), while molecular NLO responses are calculated by quantum-mechanic methods [46]

Conducting polymers are regarded as excellent third-order NLO materials due

to highly polarizabilityҏ electrons in their polymeric chain It was predicted that (3)

F is proportional to the reciprocal of the band gap raised to the sixth power [47],